Low-magnitude earthquakes in Rome: structural interpretation and implications for the local stress field

On 1995 June 12, a ML = 3.6 earthquake occurred in the Cecchignola area of southern Rome. A detailed structural investigation of this area and an analysis of macroseismic and instrumental data indicate a seismogenic area for the local low-magnitude seismicity that affects the city of Rome. A peripheral volcanic field in Cecchignola shows that the local seismicity could be related to the same volcanotectonics that affects the Colli Albani (Alban Hills) region. Fracture-field emission is proposed for the emplacement of about 2.5 km3 of lava, which developed a small plateau that was subsequently intruded by the feeder dyke of a large ignimbrite. The geometry of this dyke as well as the faults disrupting the lava field indicates NE–SW extension. Younger tectonic features show a NW-striking σ3 in the studied area, suggesting that dextral N–S strike-slip displacement generated structures that are not compatible with the local present-day stress field, characterized by a NE-striking σ3. An analysis of focal solutions and the spatial distribution of the 1995 June 12 microearthquake sequence suggests an E–W seismogenic structure whose trend is atypical of the structural setting of the area. The present study identifies surface faulting with the same trend as the focal mechanisms in the epicentral area and suggests that these faults represent an inactive surface expression of the deeper structures. It is also suggested that these structures originated during a different tectonic regime and are presently reactivated at depth by the NE–SW extension. The general NE orientation of the T axes from focal mechanisms for the Colli Albani earthquakes and for the 1995 June 12 Cecchignola event strongly supports a regional interpretation for the present-day stress field previously observed only in the Colli Albani area. It is likely that a combination of different tectonic styles during the last 600 kyr is necessary to explain the structural features of the Cecchignola area and of the Central Tyrrhenian Sea margin of Italy in general.